remove mention of a key compromise

That scenario is more complex and has more implications than just
renewing the key. It *also* includes cert revocation so it's implicit
as well.

tsa/howto/letsencrypt.mdwn

@@ -81,8 +81,8 @@ Then run Puppet on all affected hosts, for example the static mirrors:
 
 # Renewing a certificate before its expiry date
 
-If a certificate has been compromised or revoked, it should be renewed
-before its expiry date. To do so, you can drop a special file in the
+If a certificate has been revoked, it should be renewed before its
+expiry date. To do so, you can drop a special file in the
 `per-domain-config` directory to change the expiry date range and run
 the script by hand.